Add typed DPR request Client Hint metadata

[fewensa]

Summary

• Add bounded singleton DPR request Client Hint parsing with finite positive decimal validation and raw header preservation.
• Expose typed DPR metadata through client, server, and rttp facades, including an opt-in HttpClient::dpr setter.
• Add focused protocol, facade, raw request, and HTTP/1.1 matrix coverage plus metadata-only docs.

Verification

• cargo test -p rttp-protocol --test client_hints
• cargo test -p rttp_client --test metadata_facade
• cargo test -p rttp-workspace-tests --test metadata_http11_interoperability_matrix
• cargo test -p rttp-server --test metadata_facade
• cargo test -p rttp_client --test test_raw_request_capture dpr
• cargo fmt --all -- --check
• git diff --check

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-13T02:46:36.419746Z	7f0fd83	PR opened

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